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Were multiple early human species living in Georgia, 1.85 million years ago?

New skull with ‘enigmatic’ jawbone and differing tool technologies suggests that two different hominin groups are represented by Dmanisi remains.

The former Soviet republic of Georgia is located at the crossroads of Europe and Asia. Lying on the eastern shores of the Black Sea, it was the destination of Jason and the Argonauts in their quest for the Golden Fleece, but long before this it was a stopping point for the earliest-known hominin migration out of Africa. In 1984, stone tools were discovered at the small medieval town of Dmanisi in the southeast of the country, 93 km (58 miles) southwest of the capital, Tbilisi. Archaeologists broke through the foundations of a medieval building into an ancient river deposit, where simple stone tools resembling those made by the earliest humans were found with the bones of extinct mammals.

During the 1990s, the remains of early humans were recovered, including two partial skulls and a lower jawbone. The fossils were dated by palaeomagnetic, potassium-argon and argon-argon methods, giving an age for the remains of 1.77 million years old (Gabunia, et al., 2000). Subsequent dating of the stone tools indicated that the site was first occupied 1.85 million years ago, and that repeated occupations continued over a period of 80,000 years. There was evidently a long-term human presence in the Caucasus at around or even before the time of the earliest evidence for Homo erectus in Africa (Ferring, et al., 2011).

There have been a number of subsequent discoveries of human remains at the site. These include the skull, lower jawbone and partial skeleton of an adolescent (Vekua, et al., 2002; Lordkipanidze, et al., 2007); the skulls and lower jawbones of two adults (Lordkipanidze, et al., 2006; Lordkipanidze, et al., 2013); and postcranial bones from three other individuals, all adults (Lordkipanidze, et al., 2007). One of the skulls belonged to an elderly male who had lost all but one of his teeth some years prior to his death. He could not have survived unaided and must have been cared for by his companions throughout those last years of his life (Lordkipanidze, et al., 2005; Lordkipanidze, et al., 2006). The other skull, the fifth to be discovered at the site and hence known as Skull 5, is characterised by a large face and thick browridges. Skull 5 is complete and undeformed; it is the only known fully-preserved adult hominin skull from the early Pleistocene (Lordkipanidze, et al., 2013).

From the various remains, body size metrics have been estimated for the Dmanisi hominins. They were 1.45 to 1.66 m (4 ft. 9 in. to 5 ft. 5 in.) tall and weighed 40.0 to 50.0 kg (88 to 110 lb.). The cranial capacities of the five skulls range from 546 to 730 cc, about half that of a modern human. The encephalization quotient (a measure of brain size in relation to body size) lies in the range from 2.4 to 3.13; a figure that is at the lower end of the estimates for African Homo erectus, and is more comparable to that of Homo habilis or Australopithecus (Lordkipanidze, et al., 2007; Lordkipanidze, et al., 2013).

The Dmanisi hominins display a mosaic of primitive and derived (more modern) features. Their limb proportions were similar to those of a modern human. The lower limbs and feet were essentially modern, although the feet turned slightly inwards. On the other hand, the forearm lacked what is known as humeral torsion. In modern humans, the elbow joint is typically rotated relative to the shoulder joint, so that the forearm naturally hangs with the palms facing inwards; but the Dmanisi forearm lacked this rotation, so their palms were oriented more forwards. The inward-turning feet, lack of humeral torsion, small body size and small brain size may be seen as primitive traits, sharing more in common with Homo habilis than with Homo erectus (Lieberman, 2007; Lordkipanidze, et al., 2007).

Initially assigned to African Homo erectus (Vekua, et al., 2002), the Dmanisi hominins were later put forward as a new human species, Homo georgicus (Gabunia, et al., 2002); though this proposal has since been retracted (Lordkipanidze, et al., 2013), and it has been suggested that early African Homo erectus was not only quite widespread, but also unusually variable in both body and brain size, and also less modern than sometimes supposed (Lieberman, 2007).

Two more radical (and diametrically-opposed) possibilities have recently been put forward. The first is that the various species often proposed for early African Homo (Homo habilis, Homo rudolfensis, Homo ergaster and Homo erectus) were all actually variants of the same species, and that early Homo was a single lineage which evolved over time without differentiating into multiple species. This conclusion is based on a claim that shape variation between the five Dmanisi skulls is roughly the same as that seen among the various early Homo skulls from East Africa, even though the former represents a single species and the latter are generally thought to represent several (Lordkipanidze, et al., 2013).

The second proposal (Bermúdez de Castro, et al., 2014) is that Skull 5 represents a different group of early hominins to that of the other Dmanisi remains. The lower jawbone is larger than those of others, and is said to represent a ‘large and somewhat enigmatic individual’. Its shape differs, and the differences cannot be accounted for in terms of body size or sex. It possesses a mosaic of primitive and derived features that are absent from other Dmanisi specimens. Furthermore, patterns of dental wear suggest a higher intake of fibrous and abrasive foods. It has accordingly been suggested that the jawbone is adapted to a different ecological niche to the other Dmanisi hominins, and that it represents a different species.

Although tools document a long-term human presence at Dmanisi, all the actual human remains were found in the same geological layer. This makes the ‘two species’ scenario problematic, as it implies that both species lived at about the same time. However, the stratigraphy of Dmanisi is complex, and it is possible that the fossil remains were re-deposited in the same geological layer after initially occupying sediments of different ages. It has also been claimed that the tools found at Dmanisi are consistent with the existence of two different populations.

More evidence is needed to determine just where the Dmanisi hominins fit into the broader human evolutionary picture, but it is becoming clear that the first hominin dispersal out of Africa was a far more complex process than was at one time supposed.